The present invention relates to an in-plane switching type liquid crystal display apparatus. More particularly, the present invention relates to an in-plane response type liquid crystal display apparatus which is obtained by adhering two substrates of which at least electrodes on one side thereof are of a comb-like shape on at least one side thereof and enclosing liquid crystal therein, wherein it is arranged such that irregularities in colors can be eliminated and display property improved.
In contrast to twisted nematic display methods wherein an electric field is applied on liquid crystal in a direction vertical with respect to a substrate, developments are being made in these years employing display methods in which an electric field is applied in a direction substantially parallel with respect to a substrate. For instance, Japanese Unexamined Patent Publication No. 225388/1995 discloses an example of a liquid crystal display apparatus employing such a display method in which an electric field is applied in a direction substantially parallel with respect to a substrate, in which it is referred to that the gap between substrates is set to be not more than 6 .mu.m or that the retardation, which will be described below in details, to be not less than 0.21 .mu.m and not more than 0.36 .mu.m in order to improve the response speed.
The inventors of the present invention have defined the above described liquid crystal display apparatus employing a display method in which an electric field is impressed in a direction substantially parallel with respect to a substrate as an in-plane switching liquid crystal display apparatus (hereinafter referred to as "IPS panel").
FIG. 11 is a partial sectional explanatory view of a conventional in-plane switching type liquid crystal display apparatus. In the drawings, only two pixels have been partially shown for ease of explanation (and this also applies to all drawings hereafter). FIG. 12 is a plan explanatory view of one pixel in a conventional IPS panel. FIG. 13 is a sectional explanatory view taken along line X-Y of FIG. 11. In FIG. 11, FIG. 12, and FIG. 13, numeral 1 denotes a TFT array substrate, numeral 2 pixel electrodes, numeral 3 TFT portions, and numeral 4 a counter substrate. The surface of the TFT array substrate 1 is provided with electrodes which are of comb-like shape. Each of the pixel electrodes 2 is comprised of a comb-like liquid crystal driving electrode 21 and a comb-like common electrode 22 at least a part of which is formed to oppose the liquid crystal driving electrode. TFT portion 3 is a generic term for thin film transistors (hereinafter referred to as "TFT") 14 for writing image signals to the liquid crystal driving electrodes 21, and signal cables for supplying image signal cables 12, scanning signal cables 11, or common electrodes 22 with signals. The counter substrate 4 is arranged to oppose the TFT array substrate 1. Materials for forming the pixel electrodes 2 may be any one selected from among chrome, aluminum, indium tin oxide (hereinafter referred to as "ITO"). Numeral 25 denotes an insulating film. The counter substrate 4 is not required to be provided to be with electrodes on the surface thereof in case of IPS panels performing colored display, and is generally provided with a flag portion (not shown) formed of metal or resin and a color filter substrate with coloring layers 18 of red, green and blue. In order to prevent the coloring layers 18 from melting to the exterior, a protecting film 24 is formed on the counter substrate 4. Numeral 17 denotes electrodes for writing in image signals, scanning signals or common signals from the substrate exterior. Numeral 7 denotes liquid crystal, and the thickness thereof is represented as d. Further, numeral 15 in FIG. 13 denotes one molecule within the liquid crystal layer. Numeral 9 is a seal member for connecting the TFT array substrate 1 to the opposing substrate 4, numeral 10 secondary spacers contained in the seal member, and numerals 5 and 6 polarizer. Numeral 23 denotes an alignment layer for aligning the liquid crystal 7. Further, in FIG. 13, I.sub.0 denotes incident light, I outgoing transmitted light, T transmittance axis, D aligning direction, and E electric field.
The basic arrangement of the IPS panel will be explained with reference to FIG. 11. In the basic arrangement of the IPS panel, the TFT array substrate 1 and counter substrate 4 are opposed in a parallel manner, connected by seal member 9, and liquid crystal 7 is interposed between the TFT array substrate 1 and the counter substrate 4. Alignment treatments have been performed for the orientation film 23 (detailed descriptions thereof will be described later). Primary spacers 8 are dispersed within the seal surface for maintaining the gap d between the TFT array substrate 1 and the counter substrate 4 constant, and secondary spacers 10 are disposed by mixing them into the seal member 9.
While it has been defined in the prior art as disclosed in the above mentioned Japanese Unexamined Patent Publication No. 225388/1995 that the gap between the substrates shall not be more than 6 .mu.m, there is made neither any reference to secondary spacers 10 as shown in FIG. 11 which are known in the art by the inventors of the present invention nor to making the gap between the substrates constant. This is considered to be due to the fact that it had not been recognized of the connection of these with improving irregularities in colors of display which is a subject of the present invention as it will be described in details hereafter.
The operational theory of the IPS panel will now be explained with reference to FIG. 13. As noted in the above explanation of the basic arrangement, the TFT array substrate 1 and counter substrate 4 are opposed in a parallel manner. In case the liquid crystal 7 is liquid crystal of positive anisotropy of dielectric constant, liquid crystal molecules 15 are disposed such that the longitudinal axes thereof are parallel with respect to the substrate surface and such that they are also substantially parallel with respect to the electrodes. For this disposing method, known rubbing methods are generally used, and in case rubbing is performed for the alignment layer 23 in a direction substantially parallel to the electrodes, the liquid crystal molecules 15 assume the above described alignment. In this manner, the polarizer 5 is disposed such that the transmission axis thereof is parallel with respect to the aligning direction, and the polarizer 6 as to be orthogonal to the transmission axis of the polarizer 5.
In case the electric field is OFF, light that has passed through the polarizer 5 reaches the second polarizer 6 along the alignment of the liquid crystal molecules 15. Light can not path through since, as noted above, the transmission axis of the polarizer 6 is orthogonal to the transmission axis of the polarizer 5. In case the electric field is ON, that is, an electric field has been generated in a horizontal direction with respect to the substrate between the liquid crystal driving electrodes 21 and common electrodes 22 which have been formed to as to oppose the former, the longitudinal axis of the liquid crystal molecules rotates in a parallel manner with respect to the substrates along a direction of the electric field which is due to the anisotropy of dielectric constant of the liquid crystal. At this time, the transmitted light changes from a linear polarized light to an elliptical polarization (the elliptical polarization is schematically shown on the counter substrate 4 in FIG. 13 on the right-hand side thereof due to birefringence effects, and passes through the polarizer 6.
In this manner, the IPS panel employs the birefringence effect. The birefringence effect is generally called an ECB (electrically controlled birefringence) effect (hereinafter referred to as "IPS mode"). Since the liquid crystal molecules comprise a refractive index of ordinary light n.sub.o and a refractive index of extraordinary light n.sub.e, there exists an anisotropy of refractive index .DELTA.n=n.sub.e -n.sub.o. The presence of .DELTA.n in the liquid crystal molecules causes birefringence effects.
In the ISP panel, the liquid crystal molecules 15 are aligned as to be uniformly directed in one direction parallel to the substrate as shown in FIG. 13. Such a condition of alignment is called a homogeneous alignment. The intensity of transmission light I of light outgoing from polarizer 6 in case the polarizers 5, 6 have been disposed as shown in FIG. 11 by employing homogeneous alignment is given by the following equation. EQU I=I.sub.o sin.sup.2 (.pi.R/.lambda.)
where I.sub.o represents intensity of light incident on the polarizer 5, .lambda. represents wavelength, and R represents retardation which is obtained by multiplying an optical-path difference between ordinary light and extraordinary light by thickness of the liquid crystal ((.DELTA.n).multidot.d). That is, the intensity of light which is outgoing from the polarizer 6 is given by a function of wavelength .lambda., of incident light and retardation R. Thus, variations in thickness d of the liquid crystal 7 cause variations in outgoing light intensity, that is, transmission light I. PA1 a counter substrate which opposes the TFT array substrate, PA1 a plurality of primary spacers for maintaining a gap between the TFT array substrate and the opposing substrate constant, PA1 a plurality of secondary spacers for maintaining a gap between peripheral portions of the TFT array substrate and the counter substrate constant, PA1 a sealing agent which is interposed in the gap between the TFT array substrate and counter substrate together with the secondary spacers and which adhere the TFT array substrate and counter substrate at the peripheral portions, and PA1 a liquid crystal layer pinched and held between the array substrate and counter substrate, PA1 a counter substrate which opposes the TFT array substrate, PA1 a plurality of primary spacers for maintaining a gap between the TFT array substrate and the counter substrate constant, PA1 a plurality of secondary spacers for maintaining a gap between peripheral portions of the TFT array substrate and the counter substrate constant, PA1 a sealing agent which is interposed in the gap between the TFT array substrate and counter substrate together with the secondary spacers and which adhere the TFT array substrate and counter substrate at the peripheral portions, and PA1 a liquid crystal layer pinched and held between the array substrate and counter substrate,
In the theory of a conventional IPS mode as described above, variations occur in outgoing transmission light owing to variations in thickness of the liquid crystal layer. Variations in outgoing transmission light I causes variations in colors of display (hereinafter referred to as "irregularities of colors"). Measures had already been taken in the prior art for eliminating variations in thickness of liquid crystal layers, one of which is disclosed in Japanese Unexamined Publication No. 286176/1996, wherein a planation layer of transparent resin 17 is provided on the color filter for planation of the surface, and also Japanese Unexamined Patent Publication No. 225388/1995 discloses such a provision of a flatting layer on the color filter, and it is considered that such a flatting layer is effective in making the thickness of the liquid crystal layer uniform.
However, the inventors of the present invention have found out, as it will be discussed in details hereafter, that it is impossible to decrease variations in thickness of a liquid crystal layer even in a conventional arrangement in which spacers have been provided between two substrates, and much less is it possible to accurately make the thickness of the liquid crystal layer uniform by using a planation layer as disclosed in the above publications (Japanese Unexamined Patent Publication No. 286176/1996 and Japanese Unexamined Patent Publication No. 225388/1995). Problematic points of conventional IPS panels will now be explained based on FIG. 14.
There are cases in which the primary spacers 8 are either arranged on the TFT portions 3 or alternatively, on the pixel electrodes 2. Generally, the volumetric ratio of TFT portions 3 and pixel electrodes 2 of a TFT array substrate 1 is approximately 7:3. Therefore, it is often the case that primary spacers 8 on the pixel electrodes 2 determine the thickness of the liquid crystal 7. Thus, it is often the case that d.sub.max and d.sub.min exist within a single IPS panel in which the thickness of liquid crystal 7 is uniform as shown in FIG. 14. FIG. 14 is a sectional explanatory view showing problematic points of a conventional IPS panel, and reference numerals as used therein are identical with those of FIGS. 11, 12 and 13. In this description, the thickness of the liquid crystal layer of pixel portions which are governed by the primary spacers 8 in convex portion of uneveness on the TFT portions 3 is defined as d.sub.max (indicating pixels on both sides of the convex portion and the larger one is defined as d.sub.max), and the thickness of the liquid crystal layer of pixel portions which are governed by the primary spacers 8 in a concave portion of uneveness on the pixel electrodes 2 is defined as d.sub.min. The value of d.sub.max -d.sub.min of a TFT array substrate, which difference in height of the convex portions on the TFT portions 3 and the depth of the concave portions on the pixel electrodes 2 was approximately 1 .mu.m, was approximately 0.8 .mu.m.
In a general IPS mode, the retardation (.DELTA.n).multidot.d is set to be 275 nm. In our studies, it has been found that display deficiencies occurred in case retardation (.DELTA.n).multidot.(d.sub.max -d.sub.min) was different by not less than 20 nm. The .DELTA.n of liquid crystal generally used in an IPS mode is 0.05 to 0.15. That is, in case d.sub.max -d.sub.min is not less than 0.8 .mu.m, (.DELTA.n).multidot.(d.sub.max -d.sub.min) is in the range between 40 to 120 nm and thus causes display deficiencies as already described. Thus, d.sub.max -d.sub.min is required to be not more than 0.4 .mu.m in order to make (.DELTA.n).multidot.(d.sub.max -d.sub.min) to be not more than 20 nm.
As described, variations in thickness of liquid crystal 7 caused irregularities in colors in conventional arrangements of IPS panels, and thus caused deficiencies in display property of display.
Further, since difference in height of the convex portion of the TFT array substrate and the depth of the pixel electrode was approximately 1 .mu.m in a conventional arrangements of TFT array substrates 1 which was formed on the TFT array substrate 1 which was due to arrangements of the TFT portions or pixel electrodes. Thus, the spherical shape of secondary spacers 10 contained in the seal member 9 and the diameter of primary spacers 8 dispersed within the display surface could not be accurately set. Therefore, variations in dispersing processes of secondary spacers 10 and primary spacers 8 caused differences in the thickness d.sub.min of the liquid crystal 7 in the proximity of the seal member 9 and in the thickness dmax of the liquid crystal 7 in the central portion of the display surface, whereby display deficiencies were generated.
The present invention has been made with the aim of solving the above described problems, and it is an object of the present invention to provide an IPS panel of favorable display property by eliminating variations in colors which were due to unevenness in thickness of the liquid crystal layer which existed in conventional arrangements of IPS panels.